Font size change causes a progression shift.

[mauricecarrier7]

Bug Report

What happened?

When changing the font size, within a title that has images, the location shifts to the beginning of the current chapter.

changing_font-size_ios_15.mp4

Expected behavior

Chapter location should not change.

How to reproduce?

1. Open a title that has images
2. Navigate to a progression within a chapter
3. Change font to a larger or smaller size
4. See page position change

Environment

• Readium version: 2.3.0

Development environment

macOS: 12.3.1
platform: arm64
carthage: 0.38.0
Xcode 13.4
Build version 13F17a

Testing device

• iOS version: 13.0, 14.0.1, 15.0, 15.5
• Model (e.g. iPhone 11 Pro Max): 11, 11 Pro, 11 Pro Max, SE, 13 Pro
• Is it an emulator? Simulator, emulator and physical device

Additional context

• Are you willing to fix the problem and contribute a pull request? Yes

[mickael-menu]

Thank you for reporting this issue.

My guess is that this is caused by the fact we are using the scroll percentage to restore the location, for example:

1. Current location is 45%.
2. We increase the font size.
3. The navigator try to reposition to 45%, but ends up slightly before as the number of screen pages is different, for example 44%.
4. We decrease the font size.
5. The navigator try to reposition at 44%, so we end up at a different page than the one in step 1.

Here's two solutions I think could help:

• Using a text context in the current locator, to restore to a text snippet instead of scroll location.
• Keeping the current locator "frozen" while changing multiple time the user settings, defreezing it when the user actually moves to a different location.